The present invention generally relates to optical semiconductor devices and more particularly to a semiconductor photodetection device and fabrication process thereof.
With widespread use of internet, there is emerging tightness of capacity in optical telecommunication network. In order to deal with the problem, the technology of so-called wavelength-division multiplexing (WDM) is now becoming in use. In WDM, plural optical carriers of different wavelengths are transmitted via a single optical fiber, wherein the optical carriers are modulated with respective modulation signals.
In order to increase the bandwidth of the WDM channels, a study is being made to expand the wavelength range of the optical carriers to longer wavelength side, up to the wavelength of 1620 nm (so-called L-band). In order to implement this technology, it is necessary to provide a semiconductor photodetection device having sufficient sensitivity in the long wavelength range of 1620 nm.
FIG. 1 shows the construction of a conventional PIN photodiode 20 of the type that receives incoming optical signal at a rear surface.
Referring to FIG. 1, the PIN photodiode 20 is constructed on a substrate 7 of n-type InP, and includes a first buffer layer 6 of n-type InP formed on the substrate 7, a second buffer layer 4 of n-type InP formed on said buffer layer 6, a photodetection layer 3 of n-type InGaAs formed on the n-type InP buffer layer 4, a cladding layer 2 of n-type InP formed on the photodetection layer 3, and a contact layer 1 of undoped InGaAsP formed on the cladding layer 2, wherein the foregoing second buffer layer 4, the photodetection layer 3, the cladding layer 2 and the contact layer 1 form a mesa structure on the first buffer layer 6. In order to facilitate the formation of the mesa structure, there is provided an etching stopper layer 5 of n-type InGaAsP between the first buffer layer 6 and the second buffer layer 4.
The contact layer 1 and the cladding layer 2 are further formed with a p-type diffusion region 8, and a p-type electrode 9 is provided in contact with the foregoing p-type diffusion region 8. Further, an n-type electrode 10 is provided on the n-type InGaAsP etching stopper layer 5 in the part exposed as a result of the mesa formation. Further, a micro-lens 7A is formed on the bottom surface of the InP substrate 7.
In the PIN photodiode 20, the incident optical beam incident to the bottom surface of the InP substrate 7 is focused into the part located in the vicinity of the photodetection layer 3 and there occurs excitation of photocarriers in the part located in the vicinity of the photodetection layer 3. The photocarriers thus excited are then caused to flow to the p-type electrode 9 or to the n-type electrode 10 depending on the polarity and form a photocurrent.
In such a PIN photodiode 20, it is generally practiced to form the InGaAs photodetection layer 3 with a thickness of about 2 μm and with a composition chosen so as to achieve lattice matching with respect to the InP substrate 7, for minimizing the occurrence of crystal defects and for maximizing the efficiency of optical absorption. As long as the photodetection layer 3 has a thickness of about 2 μm, a satisfactory optical absorption efficiency is achieved while simultaneously suppressing the problem of carrier transit time, and a high-speed response sufficient for dealing with a transmission rate of 10 Gbit/s as used in optical fiber telecommunication network is attained.
In the PIN photodiode 20 of FIG. 1, it should be noted that the InGaAs photodetection layer shows a bandgap wavelength of about 1650 nm at room temperature in view of the necessity of achieving lattice matching with the InP substrate 7. Thus, the PIN photodiode 20 can detect the optical signal of longer wavelength of 1620 nm as long as the PIN photodiode 20 is operated in a room temperature environment. On the other hand, the photodiode 20 used in an optical fiber telecommunication network is required to guarantee a proper operation even in extremely low temperature environment as low as −40° C.
When the PIN photodiode 20 of FIG. 1 is operated in such a low temperature environment, on the other hand, there occurs an increase in the bandgap of the photodetection layer 3, and thus, the efficiency of optical absorption of the layer 3 drops below 50%. While such an increase of the bandgap can be compensated for by increasing the In content in the layer 3, such an increase of the In content in turn invites a lattice misfit between the photodetection layer 3 and the InP substrate 7. Thus, when the photodetection layer 3 is to be formed with the thickness of 2 μm suitable for optical absorption, it is inevitable that there occurs a substantial defect formation in the photodetection layer 3. When there occurs an increase of defects in the photodetection layer 3, there arises various problems including increase of leakage current and increase of dark current. In order to avoid the formation of crystal defects, it is necessary to suppress the strain accumulated in the photodetection layer 3 to be less than 0.1%. However, this requirement is contradictory to the requirement to compensate for the unwanted increase of the bandgap at low temperature environment.
In order to deal with this problem, there has been a proposal, in Japanese Laid-Open Patent Publication 7-74381 or 62-35682, to construct the photodetection layer 3 from an optical absorption layer and a strain-compensating layer disposed adjacent to the optical absorption layer and cause the strain-compensating layer to accumulate a counteracting strain (tensile strain in the present case) therein such that the strain-compensating layer and the optical absorption layer as a whole compensate for the stress caused in the photodetection layer 3 by the substrate 7. According to such a construction, it is possible to secure a sufficient thickness for the optical absorption layer by repeatedly stacking the optical absorption layer and the strain-compensating layer alternately.
For example, the Japanese Laid-Open Patent Publication 7-74381 discloses a photodetection layer 21 represented in FIG. 2 in which the photodetection layer 21 is formed of an alternate repetition of an optical absorption layer 21a having a composition of In0.53−xGa0.47+xAs and accumulating therein a tensile strain and a strain-compensating layer 21bhaving a composition of In0.53+xGa0.47−xAs and accumulating therein a compressive strain.
In the example of FIG. 2, the photodetection layer 21 is interposed between a lower contact layer 22 of n-type InP and an upper contact layer 23 of p-type InP. Further, the Japanese Laid-Open Patent Publication 62-35682 discloses a photodetection layer provided on a GaAs substrate and formed of an alternate repetition of a GaAs layer having a thickness of 10 nm and an AlGaAs layer having a thickness of 10 nm.
These conventional structures, however, have suffered from the problem in that the net thickness of the optical absorption layer in the photodetection layer becomes only one-half the total thickness of the photodetection layer.
Thus, it has been necessary to increase the number of repetitions in such a structure in order to secure a necessary total thickness for the optical absorption layer, while such an increase of the number of repetitions invites increase of the total thickness of the photodetection layer.
When the total thickness of the photodetection layer is thus increased excessively, the transit time of the carriers increases also, and the advantageous high-speed response of the conventional PIN photodiode may be lost as a result of the use of such a strained superlattice structure for the photodetection layer.